Within cellular networks, handover of a mobile station from one base station to the next occurs via either soft handoff or hard handoff.
An individual base station has finite reach and finite bandwidth that result in deployment of multiple physically or directionally separated base stations to serve large areas or where there are high counts of active mobile stations. Multiple base station signals covering a common physical point may be separated into wireless cells by frequency band, time slots, spreading codes, spatial selectivity, or combinations of these four techniques.
When an active mobile station with a single radio moves from one cell to another, the mobile station has to switch between cells while it is within the region where the two cells both have coverage. This is done in order to minimize gaps in communication. With current technologies, either the mobile station will initiate the cell association transition when communications begin to fail on the existing link, or the mobile station will be commanded by the cellular network to change at what it believes is the appropriate time to the next frequency, timeslot, spreading code, or spatial selection in order to communicate with the new targeted cell.
Depending on the radio technology selected, the mobile station may only be able to communicate to just one cell at a time. In that case the previous cell connection must be dropped before the connection is made to the new cell. These types of cell-to-cell changes are termed a hard handoff, so called because once the handover is started there is no going back—i.e., there is no “soft time” when the radio client is communicating to more than one cell where the system could decide to move back and forth several times between the cells.
Where the mobile station is managing the changeover, it can take considerable time (i.e., several seconds), especially in a crowded radio frequency (RF) band, to search for the next best cell with which to communicate. There are also smaller delays associated with negotiating with the next cell for communication rights. During this time, if the system is only capable of hard handoffs, communications are lost If the mobile station has the ability to communicate with more than one cell at a time an alternative handover technique can be used where before communications are dropped to the old cell, communications are initiated and setup completed to the new cell.
Soft handoffs are typically used on large cellular networks with cells covering many kilometers, the delay associated with searching for the next best cell is largely eliminated by having a central controller determine when is the optimum time and which cell is next to be used while the mobile client is still communicating on the old cell. The time to change cells is determined by measuring the wireless loss between the mobile client and all cell towers that detect the mobile stations signals. The relatively slow fading of the signals in such a system allows sufficiently accurate predictions as to when the mobile station will reach the effective edge of the existing and new cells. Although rough location of mobile clients is determined by the same mechanisms, the algorithms are power and time trend based rather than location based.
An example of a technology where soft handover can be done without going to the expense of having a completely duplicated radio section is Code Division Multiple Access (CDMA) where the signals from more than one cell can be separated from within in single frequency receive band. The mobile station has a single RF receiver that converts radio frequency down to baseband, but assigns a specific spreading and despreading code to each user. These codes allow the required signals to be separated from other signals by correlation.
In addition to the use of spreading and de-spreading codes commonly a “rake receiver” with multiple narrow band filters (fingers) is used. This allows strong interference signals to be eliminated but also allows more than one adjacent cell to operate on the same frequency if each cell base-stations rake settings are coordinated to be different to adjacent cells. The single RF receiver picks up all of those that are within range allowing communication to more than one cell at a time
Commonly, when the active mobile station is about halfway between two base stations, the mobile station handles transport of data back and forth to the cell along with actively looking for other cells. When the mobile station finds a base station with sufficient signal strength, the mobile station informs the network accordingly. The network might decide at that point to route the call through both base stations simultaneously such that the handoff process happens in multiple steps when the active mobile moves from one cell to another. First the mobile station notices a receiving base station, which then begins to carry the call in addition to the originating base station. As the mobile station continues to move, eventually the signal strength from the originating base station will drop to the point where it is no longer useful. Again, the mobile station will inform the network of this fact and the originating base station will be dropped in a soft transition. Thus, such a transition is termed soft handoff.
Typical soft handover systems require a centralized handoff control system with extensive knowledge and control of all base stations. This is not always practical especially when working within a large number of low cost standalone base-stations are used. Some current technologies which normally do not include a centralized handover mechanism are WiMAX, WiFi, WirelessMAN™, ZigBee™. WiFi® is a short range low cost best effort wireless data networking standard based on IEEE Standard 802.11. Typical WiFi® data rates are around 3 Mb/s but in excess of 100 Mb/s is targeted. ZigBee™ is a wireless networking standard based on IEEE Standard 802.15.4 that is aimed at remote control and sensor applications and is suitable for operation in harsh radio environments. WiMAX and WirelessMAN™ protocols are currently defined through IEEE Standard 802.16 where a wireless metro area network (MAN) provides network access to buildings through exterior antennas communicating with central radio base stations. The wireless MAN offers an alternative to cabled access networks, such as fiber optic links, coaxial systems using cable modems, and digital subscriber line (DSL) links.
Conventional handover techniques over time, allow more radio inter-cell transfers to occur than necessary and that reduces overall network efficiency. Further, conventional wireless networks utilize a large number of base station radio links where momentary radio links can cause disruptions to both data throughput and management of data flow through the network.
Loss of signal in many small cell applications can be of significant consequence. For example, real-time video feed from a mobile wireless video camera for surveillance applications can require significant amount of memory buffering and consequent latency in order to be able to prevent such loss of signal resulting in data corruptions. It would therefore be desirable to provide a mobile station handover in a small cell environment in a more predictable manner similar to soft handoff to alleviate fading problems, resultant signal degradation, or communications failure in terms of, but not limited to, complete loss of signal.
When cells become much smaller i.e. have individual cell coverage areas under a km in diameter (micro-cell), or even just a few meters in diameter (pico-cells), the radio transmissions when exposed to the typical multiplicity of dynamic elements which reflect and shadow significant amounts of RF power, the location of the usability edge of cells becomes very much more difficult to determine by conventional signal power loss techniques. In such cases, signals may fade to nothing in a fraction of a second or over just a couple of meters. Especially with micro-cell and pico-cell systems, knowing exactly where to initiate handoffs becomes critical and requires a more accurate mechanism than is currently available in large cellular systems. This applies to both systems with hard handoffs and systems with soft handoffs.